How does a rocket fly to the sky? This is a study deserving thoughtful discussion and we will introduce it to you in next units about space travel, applied and resistant force, gravitation, the air, and application of rockets. And here, we start reviewing theories and development of rockets.

In the early 13th Century, Chinese invented gunpowder and then used gunpowder to develop flying firecrackers, the predecessor of rockets. You may wonder how could easy invention, flying firecrackers, relate to rockets. In fact, the applicable theories of both are the same. Flying firecrackers take advantage of the strong thrust resulted from the burning and exploration of gunpowder. So do rockets. The only difference is rockets require a much bigger engine to escape Earth's gravity.